1. Field of the Invention
The invention relates to light emitting diode arrays. More particularly, the invention relates to circuits incorporating light emitting diode arrays which are powered by an alternating current and which are advantageously used in traffic signal and other displays.
2. State of the Art
Modern traffic signal systems include two major components: the controller and the display (lights). The technology of modern traffic signal controllers is quite evolved and includes modern computer technology which incorporates traffic flow sensors, timers, and the like. Over the last seventy years, however, traffic signal displays have not changed significantly. The displays utilize high power broad spectrum incandescent bulbs with colored filters to produce the desired traffic signal color. It is well known that traffic signal displays are inefficient, as they consume large amounts of energy in order to produce a display which is bright enough to be seen in broad daylight. The inefficiency of the displays is due in part to the general inefficiency of incandescent bulbs, and is exacerbated by the fact that much of the light energy produced by the bulbs is wasted by filtering the light. Moreover, traffic signal displays require frequent maintenance. Incandescent bulbs have a relatively short life span, typically less than eight thousand hours, shorter still if switched on and off frequently and if constantly exposed to the elements; all of which are the case with traffic signals.
It is known in the art to use a light emitting diode (LED) array in lieu of incandescent bulbs in a traffic signal. Such arrays are disclosed, for example, in U.S. Pat. No. 4,271,408 to Teshima et al., U.S. Pat. No. 4,298,869 to Okuno, and U.S. Pat. No. 4,954,822 to Borenstein, the complete disclosures of which are hereby incorporated herein by reference. An LED array can provide many advantages when used in lieu of an incandescent bulb. The primary advantages are that an LED array is much more efficient than an incandescent bulb and requires little or no maintenance. In most cases, an LED array will consume about one tenth the power that a filtered incandescent bulb will consume to produce the same light output. The life cycle costs of a traffic signal using an LED array in lieu of an incandescent bulb is also significantly reduced since incandescent bulbs used in traffic signals typically must be replaced once or twice a year. A well designed LED array could be expected to function for more than twenty years before requiring replacement. Another, less apparent advantage is that a single array can be used to display many different illuminated symbols such as international symbols for turn only, do not enter, walk, don't walk, etc. The LED array is more resistant to the elements and is more mechanically durable than an incandescent bulb. It is also possible to achieve a higher flashing rate with an LED array than with an incandescent bulb. It is known in industrial psychology that certain high flashing rates are more apt to draw attention than other slower flashing rates. In addition, an LED array does not require a light reflector like the relatively large parabolical reflectors used with incandescent bulbs. The elimination of the reflector is an advantage because during certain seasons at certain times of day, sunlight can be reflected off the reflector in an incandescent bulb traffic signal and cause a confusing display. Yet another advantage of an LED array is that, if it is properly arranged, when faults develop in the array, the entire array need not fail.
Despite all of the advantages of using LED arrays in traffic signal displays, there are several concerns which have prevented their widespread adoption. The first and perhaps the most significant concern is that an LED array is not easily retro-fitted to an existing traffic signal. This is primarily because existing incandescent displays operate with a "standard" 120 volt 60 Hz AC power supply. LEDs require a DC current of approximately 5 to 20 milliamps and a forward operating voltage of between 1.5 to 2.5 volts depending on the wavelength of the emitted light and the semiconductor material used. Another reason why retro-fitting is difficult is because the "standard" traffic signal housings are designed to accept a "standard" incandescent bulb. These issues have been addressed in the art. As shown in prior art FIG. 1, an arrangement which has been proposed by Borenstein, supra., uses a step down isolation transformer 10 with a center tapped full-wave rectifier 12 to drive an array of LEDs 14 which are connected in parallel. Although Borenstein does not specify exactly how many LEDs are to be used, a typical traffic signal display will require between twenty and eighty LEDs. Assuming that fifty LEDs are used with Borenstein's power supply, it is difficult to imagine that an efficiency of more than 50% could be achieved. Moreover, the most common LED failure mode is a short where the LED becomes a short circuit. If the LEDs are arranged in parallel as taught by Borenstein, a short fault in one LED will disable the entire array.
As shown in prior art FIG. 2, a simpler arrangement which has been proposed by Teshima et al., supra., uses a rectifier bridge 16 to convert the AC power supply to pulsating DC and an array of sixty-two 1.6 volt LEDs 18 in series with a resistor 20. A smoothing capacitor 22 is connected in parallel with the array for absorbing ripple components of the power supply. Unfortunately, the rectifier circuit adds expense to the system and makes it less reliable. The resistor wastes energy and lowers the efficiency of the system. While Teshima et al. suggests that the rectifier can be eliminated by using pairs of oppositely polarized LEDs connected in series through a protective resistor, little information is given about this arrangement.
A simpler solution has been proposed by Okuno, supra., which is shown in prior art FIG. 3. Okuno avoids the use of a rectifier bridge by providing an array of LEDs 24 which are connected in series and polarized in one direction and an array of LEDs 26 which are connected in series and polarized in the opposite direction. The two arrays 24 and 26 are connected in parallel so that a respective array is illuminated during each half cycle of the AC power supply. According to Okuno, however, a current limiting resistor 28 (a generator resistor) must be connected in series with the arrays. Assuming each array 24 and 26 includes twenty-five LEDs, the value of the resistor 28 should be approximately 3300 ohms to produce the desired average LED current. Since approximately 70% of the line voltage is dropped across the resistor 28, the resistor is the dominant factor in determining the LED current and energy is wasted by the resistor. In this example, the arrangement has an efficiency of only about 35% and the LED current has a range of .+-.25%. If a greater number of LEDS were used, the efficiency would increase, but the current range would widen.